The present invention relates to a thermoplastic resin composition capable for being used for skin materials of automotive interior parts due to excellent extrusion and calender moldability. More particularly, it relates to an olefin-based thermoplastic resin composition comprising a base composition comprising a polypropylene resin, an olefin copolymer rubber, a processing oil, and a low-density polyethylene resin or a high-density polyethylene resin; organic peroxide crosslinking agent; a crosslingking aid; and polytetrafluoroethylene resin.
Polyvinylchlorides (PVCs) or PVC/acrylonitrile-butadiene-styrene graft copolymer(ABS) alloy extruded sheets or calendered sheets have been widely used for skin materials of automotive interior parts such as instrument panel, door trim panel, headlining, and the like. Recently olefin-containing thermoplastic polyolefin (TPO) sheets have replaced PVCs which have a few drawbacks such as the difficulty in recycling and lightening, generation of environmentally unfriendly compounds, fogging phenomenon, and offensive odor. However, since a processing oil is added to assist the molding process of olefin copolymer rubbers such as ethylene-propylene-diene monomer (EPDM), the olefin-containing thermoplastic elastomer resins have poor calender moldability and a low melt strength which cause deterioration in the second moldability such as vacuum thermoforming, thus being inappropriate to use as automotive parts requiring deep drawing. When an amount of the processing oil used is reduced and a degree of crosslingking of EPDM is increased to improve the calender moldability, the molded articles have a very rough surface. On the other hand, if the degree of crosslinking of EPDM is lowered, the obtained resin composition sticks to the surface of rolls at the time of molding process.
To solve such problems, U.S. Pat. No. 4,212,787 discloses that pre-crosslinked EPDM rubber is diluted into a polypropylene resin having good fluidity. However, practically sufficient elasticity and mechanical properties cannot be obtained to use for complicated articles such as instrument panel and door trim panel because the level of crosslink density is low. Further, it requires special facilities for crosslinking process and dilution process and the manufacturing cost is also increased. JP Unexamined Publication No. 48-26835 discloses that a polypropylene resin and EPDM rubber are pre-mixed and melt-kneaded and then it is heat-treated with peroxide. However, practically sufficient degree of crosslinking and viscosity to use for calender molding cannot be obtained due to hyper-degradation of the polypropylene resin and local crosslinking of EPDM rubber. U.S. Pat. No. 4,311,628 discloses a method of homogeneous crosslinking of EPDM rubber using a phenol-base crosslinking agent. The resin prepared by this method exhibits better elasticity and mechanical properties due to favorable degree of crosslinking and homogeneous particles compared to the resin prepared by using peroxide-base crosslinking agent but there are drawbacks in that there are problems associated with prolonged crosslinking time, use of an expensive crosslinking agent raises the cost of final products, and the resin has poor solubility, resulting in decrease of coatability. Further, U.S. Pat. No. 4,212,787 discloses a peroxide-non-curable-hydrocarbon rubbery material such as polyisobutylene (PIB) and butyl rubber (BR) is used when EPDM/polypropylene composition is cured and kneaded with peroxide to improve the fluidity without degrading properties of the thermoplastic elastomer composition. However, even if the thermoplastic elastomer composition exhibits homogeneous crosslink density of EPDM, the crosslink density of the final products are not sufficient for calender molding. Therefore, the composition is poor in mechanical strength, moldability, permanent compression set and heat resistance.
In order to solve the above problems, we developed a novel olefin-base thermoplastic resin composition that maintains the crosslink density of an olefin copolymer rubber. This is accomplished in part by adding an organic peroxide crosslinking agent, a crosslinking aid, low density or high density polyethylene resin, and polytetrafluoroethylene (PTFE) in an appropriate amount during the crosslinking process of a blend of a polypropylene resin, an olefin copolymer rubber and a processing oil.
Accordingly, an object of the present invention is to provide a thermoplastic resin composition capable of the second molding for automotive parts having high deep drawing such as instrument panel as well as calender and extrusion molding.